The present invention relates to an electric discharge machining control circuit in a wire cut electric discharge machine for performing electric discharge machining using a wire electrode. The electric discharge machining control circuit has a specific circuit for controlling a second cut after a first cut.
In a wire cut electric discharge machine, a predetermined voltage is applied to a gap formed between a wire electrode and a workpiece so as to cause a discharge between the wire electrode and the workpiece, thereby cutting the workpiece by a small amount each time via the discharge energy. Conventionally, after the first cut has been performed, the second cut is performed to correct the shape of the workpiece, smooth any rough surface, and remove any irregularities caused by machining during the first cut. In order to smooth a roughened surface, a low discharge energy is preferably used. When a low discharge energy is used, the shape of the workpiece can be easily corrected. However, when a thick workpiece is machined with low discharge energy, a machined wall surface of the workpiece is curved, thereby resulting in a loss of linearity. For example, when a workpiece having a thickness of 60 mm is subjected to the second cut, an internal portion is removed by 20 to 30 .mu.m from the edge of the machined wall surface, thereby often resulting in a loss of linearity of the cut surface.
When the discharge energy is increased, the linearity of the cut surface can be improved. However, the wire vibrates due to a discharge pressure and moves away from from the machined wall surface. As a result, sufficient shape corection cannot be performed.
For this reason, extensive studies and experiments have been made on the second cut process. It was found that the above drawback could be eliminated by decreasing an average machining voltage during the second cut operation.